In conventional incandescent lamps, a filament is provided between two electrical contacts, and current is passed between the contacts through the filament. The electrical resistance of the filament material generates heat in the filament. Typical filaments in incandescent lamps operate between about 2500 K and about 3000 K. The heated filament emits electromagnetic radiation over a range of wavelengths, some of which are within the visible region of the electromagnetic spectrum. The emittance of conventional filaments at a given temperature may be approximated by Planck's equation for black body radiation.
Conventional incandescent lamps, while providing high quality, inexpensive lighting, are extremely inefficient. Only about five to ten percent of the energy supplied to a filament is converted into electromagnetic radiation at wavelengths within the visible region of the spectrum (i.e., about 380 nm to about 780 nm). A large amount of energy is converted to radiation in the infrared region of the spectrum (i.e., between about 780 nm to about 3000 nm), and wasted as heat.
From the time incandescent lamps were first invented by Thomas Edison, significant research has been conducted to find new methods, materials, and structures to increase the amount of electromagnetic radiation emitted in the visible region of the spectrum and minimize the amount of radiation emitted outside the visible region, thereby improving the efficiency of the lamp.
Tungsten, since its first use as an incandescent filament in 1911, continues to be the material of choice as a result of its emissive properties. True black bodies do not exist in nature. However, the radiation properties of materials may be described by including factors or variables for the material's emissivity into Planck's equations for black body radiation. Emissivity is the ratio of the spectral radiant emittance (i.e., emitted power per unit area per unit wavelength) of a material to the theoretical spectral radiant emittance of a true black body. The emissivity for a given material is not constant and may vary with wavelength, the angle of observation, and the temperature of the material. The emissivity of tungsten varies with wavelength and is higher in the visible region of the electromagnetic spectrum than in the infrared region (i.e., it radiates more electromagnetic radiation in the visible region than a true black body), which makes it the material of choice for use in incandescent lamps.
Other inventions directed to increasing the efficiency of incandescent lamps include coiling the filament into coiled structures, and filling the bulb of the lamp with halogen gas. In addition, coatings of materials that are transparent to radiation in the visible region, but reflective to radiation in the infrared region, have been applied to the bulb of incandescent lamps to reflect infrared radiation emitted by the filament back onto the filament itself, thereby further heating the filament.
Recently, the use of photonic crystals as incandescent emitters has been investigated. Photonic crystals are structures comprising at least two materials having different dielectric constants interspersed periodically throughout the structure. Photonic crystals may not emit radiation continuously over a range of wavelengths when the crystal is heated, as does a classical black body. Photonic crystals may emit strongly at certain wavelengths, but only weakly, if at all over a range of wavelengths at which the crystal would be expected to emit if it were a classical black body.
Although the efficiency of incandescent lamps has been improved over time, there remains a significant quantity of energy that is emitted as electromagnetic radiation outside the visible region of the spectrum. This energy is wasted and contributes to the inefficiency of conventional incandescent lamps.